(R)-benzhydrylsulphinylacetamide, also defined as (R)-modafinil or Armodafinil, having the following formula (I)

is a compound endowed with central nervous system stimulating activity. Its preparation as described by U.S. Pat. No. 4,927,855 comprises reacting (±)-benzhydrylsulphinylacetic acid with (−)-α-methylbenzylamine (i.e. (S)-1-phenylethylamine), converting the resulting salt (S)-1-phenylethylammonium (R)-diphenyl-methanesulphinyl-acetate to (R)-benzhydrylsulphinylacetic acid, and subjecting the resulting (R)-benzhydrylsulphinylacetic acid to amidation reaction with gaseous ammonia.
In particular, preparation of (S)-1-phenylethylammonium (R)-diphenyl-methanesulphinyl-acetate, having m.p. 148-150° C., is described in step a) of preparation 1 of the above patent. It will be appreciated that such step suffers, among the others, from the following drawbacks:                the reaction mixture is very diluted, in fact the ratio between the amount of water and the amount of (±)-diphenyl-methanesulphinyl-acetic acid is about 28.7. Because of its great volume, when carried out in large scale production, the reaction needs large reaction vessels, thus affording a volumetric productivity not higher than 6%; and        to achieve the required enantiomeric purity degree, the resulting (R)-benzhydrylsulphinylacetic acid needs to be subjected to two recrystallizations, which are very time consuming.        
Similarly, Tetrahedron: Asymmetry, 15 (2004) 1053-1058 describes the preparation of anhydrous crystalline (S)-1-phenylethylammonium (R)-diphenyl-methanesulphinyl-acetate.
The inventors of the present invention have surprisingly found that such drawbacks can be overcome by making use of a different reaction medium, thus affording also a novel crystalline hydrate form of (S)-1-phenylethylammonium (R)-diphenyl-methanesulphinyl-acetate, which allows to obtain (R)-modafinil with a greater purity degree.